Devices called pigs are used to clean piping systems for removal of materials that affect flow efficiency or removal of materials that can be corrosive to the system. Pigs use an oversize fit to create a seal so that it can be pushed through the system by gas or liquid (i.e. fluid) pressure. In most cases the product in the system is used to propel the pig. The oversize interference fit provides a cleaning action.
In contemporary piping systems there has been a trend to use high yield strength thin wall pipe for buried pipe, as is allowed by piping codes, because it is less expensive to build a system with this pipe than lower yield strength heavy wall pipe. Where the system comes above grade at the inlet and outlet end, heavier wall pipe is used according to accepted design codes and corrosion allowance. Heavier wall pipe is often used when a buried system crosses beneath roads, rivers, streams and lakes.
The result of this mix of pipe is a change in internal diameter. Pipe outside diameter of any nominal size remains the same as is standard in the pipe industry. The internal diameter reduces with an increase in wall thickness. Conventional pigs utilizing cups, discs, scrapers and brushes with a central post or mandrel can be designed to flex or move to accommodate internal diameter changes where piping system design allows for their long length.
Spheres or balls are used for pigging piping systems where the shape of fittings only allow a spherical pig to pass through. For example, systems that have many branch lines often are collected into a header system that uses tee connections between the branch line and the header. The only shape that can change direction laterally in a round pipe is a sphere. In addition, some pig launching and receiving devices are designed to accept spherical pigs only.
Sphere pigs are presently manufactured as solid elastomer, foam core with an elastomer skin or wall, and hollow and inflatable elastomer. All have disadvantages.
Solid spheres do not reduce diameter easily when required, and the smooth surface does not have any scraping ability. Because solid elastomer sphere pigs do not compress easily when there are internal diameter changes, they must be sized for piping at the launch and receive end of a piping system for the ability to manually insert the pig into the heavier wall pipe and for the ability to move through the system with reasonable pressure. This then makes the sphere too small to seal or clean in typical thin wall pipe sections where the internal diameter increases. Further, a smooth or relatively smooth spherical shape has no scraping edges to allow effective cleaning.
The elastomer skin of foam core sphere pigs easily cuts and tears which reduces usable life. The foam core which is open cell can absorb product from the system which can be toxic and/or volatile, making them dangerous to handle or transport. Inflatable sphere pigs are filled with liquid (usually a water and glycol mix) to inflate them to the required diameter. Once the sphere is inflated it is not flexible for change in internal diameter.
Inflatable spheres are complicated and expensive to manufacture, requiring inflation valves to be manufactured into the sphere and special, expensive equipment is required to inflate them. If the elastomer shell or wall should be cut or tear, the sphere will deflate and lose the seal required for travel through the pipe.
None of these spherical designs have deep scraping edges perpendicular to the pipe wall to make them effective scrapers. Some spheres have wire brush strips bonded to the outside surface for scraping but they lose the pressure seal through the bristles which can cause them to slow or stop. The bristle strips also add structure so the sphere it is even more firm and will not reduce for pipeline diameter changes.
There is therefore a need for an effective sealing and scraping spherical pig for use in piping systems designed to only allow for spheres, or because of operator preference. The pig should be generally spherical to fit or travel through pipe anomalies (e.g. elbows, tees, etc.), it should transition through internal diameter changes while still maintaining a pressure seal and yet should provide aggressive scraping action.